Support relative paths

[Bartleby2718]

Filing a separate issue (per #111 (comment)) to avoid a scope creep

[Bartleby2718]

Sample LINQPad code on Windows #116):

void Main()
{
	Command.Run(
		@"C:\Program Files\dotnet\dotnet.exe",
		new[] { "--version" })
		.GetOutputAndErrorLines()
		.Dump();
}

but the following fails:

void Main()
{
	Command.Run(
		@".\dotnet.exe",
		new[] { "--version" },
		o => o.WorkingDirectory(@"C:\Program Files\dotnet\"))
		.GetOutputAndErrorLines()
		.Dump();
}

[Bartleby2718]

@madelson This doesn't have to be an option, right?

[Bartleby2718]

^A pull request has been staged. Will file a PR in this repo if all tests pass and the Mono PR is merged.